Method of anti-corrosive treatment on the surface of iron and steel materials



United States Patent 3,484,347 METHOD OF ANTI-CORROSIVE TREATMENT ON THE SURFACE OF IRON AND STEEL MATERIALS Hidehisa Yamagishi, Yokohama-shi, and Masao Takeuchi, Zushi-shi, Japan, assignors to Nippon Kokan Kabushiki Kaisha, Tokyo, Japan, a corporation of Japan No Drawing. Filed Aug. 29, 1967, Ser. No. 663,948 Int. Cl. C23b 11/00 U.S. Cl. 204-56 3 Claims ABSTRACT OF THE DISCLOSURE A thin filmy chemical coating on the surface of iron and steel materials, having excellent anti-corrosive and paint adhesive properties, can be obtained by performing an electrolytic treatment on said surface using an electrolytic solution which contains a hexavalent chromium compound as a usual main agent and a thiocyanate compound as a novel promoting agent.

This invention relates to a method of anti-corrosive treatment on the surface of iron and steel materials, and especially to an improved method of electrolytic treatment on said surface using an electrolytic solution of hexavalent chromium compound.

Among the iron and steel materials, steel thin plates have particularly wide uses in such fields as various types of structural materials and electric instruments for domestic use ormiscellaneous cans, and most surfaces thereof are coated with various kinds of paints for the combined purposes of corrosion resistance and good visual appearance. The process for common practice of such coating comprises cleaning the surface of steel plate, forming an anti-corrosive filmy layer thereon by some chemical treatment, and thereafter applying any suitable paint on said filmy layer.

In the known methods of forming the anti-corrosive chemical film on the surface of iron and steel, the most widely employed one consists of treating with an aqueous solution of chromic acid or any chromate. For instance, there is one such method which comprises coating the surface of steel with an aqueous solution of chromic acid and some reducing agent such as glycol or cane sugar, and baking the coated steel at a temperature of from 100 to 150 C., thereby forming a thin filmy coating wherein hexavalent chromium is reduced to trivalent chromium.

There is another method which comprises depositing a hydrated chromium oxide layer on the surface of steel plate by cathodic treatment of electrolytic reduction in an aqueous solution of hexavalent chromium compound and some of promoting agents such as halogen ion, aliphatic carboxylic acid or maleic acid.

Moreover, the surface of steel plate which has been subjected to such anti-corrosive processing is further coated with an appropriate paint for the double purposes of corrosion resistance and good visual appearance. However, the condition in which a paint has been coated on said surface is largely affected by the nature of the anti-corrosive filmy layer formed already as an undercoating, so that it is not always easy to obtain a strongly adhesive and stable painting. That is to say, when the anti-corrosive chemical treatment is carried out intensively so as to obtain a high corrosion resistive coating, the adhesivity of the paint to the thin layer already formed is inevitably reduced, thus causing the paint coating to come off easily. On the contrary, it is necessary to perform the anti-corrosive chemical treatment to a rather limited extent when the adhesivity of the painting thereon 3,484,347 Patented Dec. 16, 1969 is required to be increased. As an extreme case, if the paint is directly coated on the plain surface of the steel plate which has no anti-corrosive chemical layer, the adhesivity of the paint coating thereon becomes greatest of all displaying a remarkably reduced anti-corrosiveness.

It is the present status of affairs that, it is not easy to get a steel surface which is excellent in both adhesivity of paint anti-corrosiveness, since a proper balance between two opposite factors as mentioned above can not be determined without difliculty.

An object of the present invention is to remove the above-mentioned difficulty.

Another object of the present invention is to obtain a chemically treated thin layer on the surface of iron and steel materials by a simple means which will enable the painting to be applied thereon with high corrosion resistance and high adhesiveness.

The method according to the present invention has been accomplished by improving the aforementioned usual electrolylic reducing method using an aqueous solution of hexavalent chromium compound and some of promoting agents, and is characterized by using thiocyanate as a novel promoting agent.

Among various hexavalent chromium compounds available for the main agent in the electrolyte chromic acid is the most typical and economical one, which is now widely used as the anti-corrosive agent in the processing of iron and steel, and it is also the same in the method of the present invention.

When using chromic acid as the main agent in the electrolytic solution, the concentration of it in the aqueous solution may be freely selected in the wide range of from 20 to 300 grams per liter of the solution. The outstanding feature of the present invention lies in the fact that, when the surface of iron or steel material is electrolytically processed using such chromic acid or corresponding chromate solution to which a small amount of thiocyanate is added as a novel promoter, the adhesivity of the painting thereon becomes always excellent regardless of the composition or thickness of gained filmy layer. The proper concentration of thiocyanate in the electrolyte is in the range of from 6 to of the above described concentration of chromic acid chromate.

Sufficient electric energy required for the electrolytic reduction of hexavalent chromium to trivalent one on the surface of iron or steel materials in the present invention is also in the range of from 50 to 30) coulombs/d-m. as usual. However, it is advisable to use a temperature of electrolytic reduction ranging from 45 to 60 C., and the temperature of below 45 C. makes it difficult to produce a compact thin layer. Referring to the processing time, a minimum of three seconds will be sufficient to obtain an effective layer, when the electric current density is set at about 20 a./dm. It will be noted that, although the increased processing time will improve the corrosion resistance of the paint coating applied subsequently thereon, the coating tends to be somewhat reduced in its adhesivity.

When the surface of iron or steel materials is coated with a filmy layer produced by the electrolytic reduction using a electrolyte of hexavalent chromium compound containing a small amount of thiocyanate, it still remains to be seen why the subsequent paint coating can be applied with remarkably improved adhesivity. The reason may probably be accounted for by assuming that, the thiocyanate combines with the chromate ion to form a complex compound, thereby rendering the thin layer during its formation by electrolytic reduction. far more homogeneous than the case of non-use of thiocyanate, resulting in that the excellent filmy layer of high corrosion resistance and high paint adhesivity can be obtained.

The steel plate, the surface of which has been chemical- 1y processed by the commonly used method, is subjected to reduce the adhesivity of paint coating applied thereon depending on the kind of the paint used. In other words, the steel plate processed by the known art is, so to speak, very selective about the kind of paints, thus causing many difficulties in their application. However, the steel plate chemically processed by the method of the present invention displays no such selectivity. For instance, where coated with a paint based on melamine resin, it may be generalized that the steel plate chemically processed by the known art does not allow the paint coating applied thereon to present good adhesivity in the cross cut test, Erichsen test or impact test. On the contrary, the plate processed by the method of the present invention enables the paint coating applied thereon to display almost as good adhesivity as the one directly applied on the raw surface of cold rolled steel plate.

The present invention will be understood more clearly with reference to the following examples. It should be understood, however, that they are oifered only by way of illustration and are not intended to restrict the scope was processed similarly at a temperature of 60 C. for 10 seconds with a current density of a./dm. using an electrolytic aqueous solution containing 200 g. of chromic acid anhydride and 4.0 g. of sodium thyocyanate per liter of the solution. There was obtained a slightly bluish, homogeneous and anti-corrosive layer on the surface of the steel plate.

The chemically processed steel plate of the foregoing four examples were coated each with a melamine-based white paint, and the coating thus formed were subjected to the bending test, the cross cut test, the Erichsen test and the impact test about their adhesivity. Almost the same values were observed in each of said tests. The average results are presented in Table 1 in comparison with those of the paint coating directly applied on a cold rolled steel plate, and also with those of the coating performed on a steel plate which has been processed chemically by the common art not involving the use of thiocyanate. The results are expressed in multiples of a count of one by way of simplifying comparison, using those ofdirect coating at the standard.

TABLE 1.COMPARATIVE RESULTS ABOUT THE TESTS OF ADHESIVITY and breadth of the present invention or to limit the scope of claims.

EXAMPLE 1 A cold rolled steel plate containing low carbon of 0.25 mm. thickness was first electrolytically degreased in an alkaline aqueous solution, followed by water washing, and then subjected to electrolytic pickling using sulfuric acid followed by water washing. Immediately afterwards, it was chemically processed by electrolytic treatment at a temperature of 55 C. with an electrolyte containing 100 g. of chromic acid anhydride and 1.0 g. of sodium thiocyanate per liter of the solution. The electric current density at the cathode plate was 20 a./dm. and the processing time was 6 seconds. The layer formed was glossy and tinted a little bluish.

EXAMPLE 2 Immediately after the same steel plate as in Example 1 was preliminarily treated in the same manner, the plate was processed similarly at a temperature of 50 C. for 7 seconds with a current density of a./dm. using an electrolyte containing 20 g. of chromic acid anhydride and 0.1 g. of ammonium thiocyanate per liter of the solution. There was obtained a slightly yellowish and homogeneous anti-corrosive layer on the surface of the steel plate.

EXAMPLE 3 Immediately after the same steel plate as in Example 1 was preliminarily treated in the same manner, the plate was processed similarly at a temperature of C. for 2 seconds with a current density of 44 a./dm. using an electrolytic aqueous solution containing g. of chromic acid anhydride and 0.3 g. of sodium thiocyanate per liter of the solution. There was obtained a slightly bluish, homogeneous and anti-corrosive layer on the surface of the steel plate.

EXAMPLE 4 Immediately after the same steel plate as in Example 1 was preliminarily treated in the same manner, the plate The standards of evaluating mark used in the tests of adhesivity of coating in Table I are on the following bases.

Bending test:

No abnormality on the surface of coating 20 Somewhat wrinkled in the bended part 16 Remarkably wrinkled in the bended part 12 Somewhat cracked on the coating 8 Occurrence of peeling off of the coating 2 Cross cut test:

100 checkers of non-peeled coating 20 99 through of non-peeled coating 16 89 through 80 of non-peeled coating 12 79 through 50 of non-peeled coating 8 Less than 50 checkers of non-peeled coating 2 Erichsen test 7.01 mm. and over 10 From 7.00 to 6.01 mm. 8 From 6.00 to 5.01 mm. 6 From 5.00 to 3.01 mm. 4 Less than 3.01 mm 1 Impact test:

No abnormality 10 Occurrence of swelling of coating 8 Occurrence of cracks in the part of coating 6 Occurrence of cracks all over the coating 4 Peeling off of the entire coating 1 On the other hand, the rusting tests were performed under the condition of the temperature of 50 C., the related humidity of 100% and the treating time of 8 hours about an electrolytically processed steel plate by the method of the present invention and another piece of steel plate similarly processed by the prior art. Concerning the former test, only a few rusty points were observed on the surface of the plate, whereas innumerable rusty points were observed on the latter test.

Furthermore, the similar rusting tests were continued for 24 hours, and the steel plate processed by the method of the present invention only showed an increase of about 30 percent the number of rusty spots more than before,

and were substantially free from loss of brilliant gloss on the processed surface. However, the other plate processed by the prior art became notably rusty all over the surface, resulting in complete disappearance of gloss from the processed surface.

While the invention has been described in connection with some preferred embodiments thereof, the invention is not limited thereto and includes any modifications and alternations which fall within the true spirit and scope of the invention as defined in the appended claims.

What is claimed is: a

1. In the method of anti-corrosive treatment of the surface of iron and steel materials which comprises depositing a thin layer of hydrated chromium oxide on the cathodic surface of said materials by electrolytic reduction in an aqueous solution of hexavalent chromium compound, the improvement which consists of adding an inorganic thiocyanate as a promoter to the solution.

2. The method according to claim 1 wherein the hexavalent chromium compound is chromic acid, and the promoter is the one selected from the group consisting of sodium, potassium and ammonium thiocyanate.

3. The method according to claim 2 wherein the con eentration of thiocyanate in the electrolytic solution is in the range of from 0.02 to 0.005 times that of chromic acid.

References Cited UNITED STATES PATENTS 2,976,193 3/1961 Pimbley l48-6.21 2,898,250 8/1959 Pimbley a 148-6.21 2,746,915 5/1956 Giesker 20456 FOREIGN PATENTS 745,042 2/1944 Germany.

HOWARD S. WILLIAMS, Primary Examiner R. L. ANDREWS, Assistant Examiner 

